The Effectiveness of COVID-19 Vaccines During the Pre-Omicron and Omicron Periods: A Retrospective Test-Negative Case-Control Study

Abstract

Background: The aim of this study was to estimate the effectiveness of original and bivalent COVID-19 vaccines in reducing COVID-19-associated hospitalizations among the adult population of Turin, Italy.

Methods: We conducted a retrospective, test-negative, case-control study of 5768 adults aged ≥50 years who had symptoms that were consistent with COVID-19-like illness and were admitted to the hospitals of the Turin Health Unit network from 1 January 2021 to 31 January 2023. We evaluated the effectiveness of the vaccines that at the time of the study were authorized in the European Union (original/bivalent BNT162b2; original mRNA-1273; ChAdOx1-S; Ad26.COV2.S) by comparing the odds of a positive test for SARS-CoV-2 in vaccinated patients with the odds of a positive test in unvaccinated patients. The association between vaccination status, hospitalization, ICU admission and positive SARS-CoV-2 test was estimated by building multivariate adjusted logistic regression models.

Results: During the predominance of the pre-Omicron variants, the vaccine effectiveness of two and three doses received in the last 120 days against COVID-19-associated hospitalizations was 93.6% (95% CI: 90.1 to 95.9) and 97.1% (95% CI: 90.8 to 99.1), respectively. During the predominance of the Omicron variant, the vaccine effectiveness of two and three doses was 26.6% (95% CI: -0.6 to 46.5) and 75.2% (95% CI: 68.1 to 80.7), respectively, and it rose to 88% (95% CI: 78.2 to 93.3) for four or five doses of the bivalent vaccine.

Conclusions: Our study confirms that the COVID-19 vaccines protect adult patients from hospitalizations, including the subgroup ≥80 years, also during the period of the Omicron variant's predominance.

Keywords: COVID-19; bivalent vaccine; booster; effectiveness; test-negative design; vaccination.

Figure 1

Estimates of average vaccine effectiveness (VE) against COVID-19-associated hospitalization among adults aged ≥50 years during predominance period of SARS-CoV-2 pre-Omicron variants (1 January to 15 December 2021), overall and by age group. COVID-19, coronavirus disease 2019; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2; CI, confidence interval.

Figure 2

Estimates of average vaccine effectiveness (VE) against COVID-19-associated hospitalization among adults aged ≥50 years during predominance period of SARS-CoV-2 Omicron variants (16 December 2021 to 31 January 2023), overall and by age group. COVID-19, coronavirus disease 2019; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2; CI, confidence interval.

Figure 3

Estimates of average vaccine effectiveness (VE) against COVID-19-associated ICU admission among adults aged ≥50 years during predominance periods of SARS-CoV-2 pre-Omicron and Omicron variants (1 January to 15 December 2021 vs. 16 December 2021 to 31 January 2023). * Excluded from regression analysis due to complete separation (10 out of 10 test-negative controls). † Excluded from regression analysis due to complete separation (14 out of 14 test-negative controls). COVID-19, coronavirus disease 2019; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2; ICU, intensive care unit; CI, confidence interval.